As known in the prior art, analog sampled data delay lines such as semiconductor charge transfer device delay lines, more particularly charge coupled devices (CCD), can be designed in various configurations for use as signal processors, such as transversal filters. A particularly useful form of a semiconductor transversal filter device is a low pass (baseband) transversal filter in a split electrode configuration, for use in voice communications. Typically, the passband in such a case is the range of frequencies from 0 to slightly less than 4 kHz, typically 3.2 kHz, that is, the range of the most useful portion of the human voice spectrum from the standpoint of communications. Such a CCD filter typically is driven by a clock pulse sequence of clock cycle frequency of 32 kHz. Accordingly, as known in the art, since this CCD transversal filter samples the incoming electrical signal to be filtered at a 32 sampling rate, it follows that this CCD filter will necessarily also pass all frequencies in the "reflection" portion of the spectrum, that is, in the range between 28 kHz(=32 kHz-4 kHz) and 36 kHz(=32 kHz+4 kHz). This phenomenon, called "aliasing", is undesirable in that it enables the passage of unwanted "reflection" frequencies through the CCD transversal filter.
Suppression of the above-described aliasing in a CCD signal processor can be accomplished by means of an auxiliary pre-filter, for example, an auxiliary CCD transversal pre-filter driven by a clock at 64 kHz, that is, just twice that of the low pass CCD filter, and designed to have a low pass baseband, typically 0 to 4 kHz, plus a stopband typically 28 kHz to 36 kHz, with frequency roll-off therebetween, that is, passing frequencies at or below 4 kHz and increasingly suppressing frequencies above 4 kHz, so that all frequencies in the band between 28 kHz and 36 kHz are virtually completely suppressed ("rejected" or "stopped"). However such an auxiliary CCD pre-filter would require a complete set of clock circuits, input circuits, and detection circuits, all operating at 64 kHz. This would make it economically unattractive, although it would have the advantage of scaling automatically with the clock frequency of the low pass CCD filter. On the other hand, although various arrangements of RC type pre-filters could be used for an auxiliary anti-aliasing filter, such arrangements tend to be complex and as costly as the low pass CCD filter itself. Accordingly, it would be desirable to have a relatively simple pre-filter device to suppress aliasing in a CCD delay line or, more generally, in a sampled data delay line which operates on analog sampled inputs. At the same time, it would be desirable that the frequency roll-off characteristic of this pre-filter device should scale with the operating ("driving") clock frequency of the CCD itself, so that the overall device (pre-filter plus CCD) can readily be used at different clock rates.